DESCRIPTION (from the applicant): FIC1 disease includes a spectrum of liver problems ranging from benign recurrent intrahepatic cholestasis (BRIC) to severe, unremitting and histologically progressive cholestasis (Byler Disease). Systemic manifestations involve the pancreas, lung, gastrointestinal tract and auditory system. FIC1 disease result from mutations in the FIC1 (ATP8B1) gene, whose function remains elusive. We have made important advances in unraveling the pathophysiology of FIC1 disease. Farnesoid X-Receptor (FXR) mediated signaling is abnormal in ileum of children with FIC1 disease. We have modeled FIC1 disease in cell culture. The transcriptional effects of reduced FXR activity due to FIC1 deficiency include enhanced intestinal and diminished canalicular bile acid transport. We are the first to show that milder forms of FIC1 related disease (i.e. BRIC) result from a partial defect in both FIC1 function and its membrane targeting, while severe disease is associated with a complete absence of function. We have mechanistically linked FIC1 and FXR. FIC1 activates a pathway involving Phospholipase D2 (PLD2) and Protein Kinase C (PKC) ? signaling pathway leading to phosphorylation, nuclear localization and activation of FXR. This R01 proposal seeks to more fully explore the molecular mechanisms of FIC1 function and disease. Novel insights will be generated from these studies that will further our understanding of cholestatic liver disease, normal hepatic and intestinal physiology, the protean nonhepatic manifestations of FIC1 disease, and the biological and clinical consequences of FIC1 trafficking. It is very likely that these insights will translate into therapeutic advances in a broad range of disciplines. This research will involve three specific aims that test the hypothesis that plasma membrane FIC1 signals through PLD2 and PKC? leading to numerous alterations in the expression and phosphorylation of key signaling molecules including FXR. It will also test the hypothesis that in humans FIC1 disease is the result of critical changes in FXR-mediated signaling pathways. These hypotheses will be tested with the following specific aims; 1) Characterization of the signal transduction pathways influenced by FIC1, 2) Analysis of the molecular mechanisms of FIC1 disease, 3) Examination of FXR signaling in gallbladder in human cholestasis. FIC1 disease (aka Byler Disease or PFIC1) is a complex multisystem disorder whose mechanism(s) are not known. Recent molecular discoveries provide an opportunity to understand this disease (providing new therapies) and to explore basic physiology of the liver, pancreas, gastrointestinal tract and lungs.